Selenium is a trace element which is necessary for normal cellular growth and development in vivo and in vitro. In large amounts, it is a highly toxic substance; but at lower levels, it is an essential trace element in eukaryotes and a potent anticarcinogenic agent in a variety of animal models. Selenium deficiency in livestock results in degeneration of skeletal and cardiac muscle, while in humans it is associated with the development of cardiomyopathy. Dietary selenium affords protection against both the initiation and promotion of carcinogensis, and there is increasing epidemiological evidence to support its anticarcinogenic role in humans. (Medina, D. et al. (1988) Pathol. Immunopathol. Res. 7:187-199; Morris, J. G. (1984) Science 223:491-493; Batist, G. (1985) Cancer Res. 45:5900-5903; Ip, C. (1986) Adv. Exp. Med. 206:431-447; Holmberg, S. B. and Bertram, J. S. (1991) Nord Med 106:86-89).
Selenium is incorporated directly into proteins such as glutathione peroxidase, type I iodothyronine deiodinase, and selenoprotein P during translation. Incorporation of selenium into these proteins is via the selenocysteinyl-tRNA which recognizes the specific UGA codons in mRNAs to insert selenocysteine into the primary structure of selenoproteins. The biological role of selenium was established following the discovery that it is a structural component of the active center of the enzyme glutathione peroxidase.
Cellular glutathione peroxidase was the first enzyme recognized as a selenoprotein. This enzyme is composed of four apparently identical subunits, each containing one atom of selenium, and it acts as a catalyst in the reduction of hydrogen peroxide and a variety of organic hydroperoxides by glutathione. The selenium-dependent glutathione peroxidase (GSHPx) family has at least four members that protect against the deleterious effects of peroxides and hyperperoxides in tissues. These include the cellular GSHPx glutathione: H.sub.2 O.sub.2 oxidoreductase, the plasma GSHPx, the phospholipid hydroperoxide glutathione peroxidase, and the gastrointestinal tract associated GSHPx-GI. Several other selenoproteins have been characterized, including type I iodothyronine 5'-deiodinase which catalyzes the deiodination of L-thyroxine to the biologically active hormone 3,3',5-triiodothyronine, and the T-cell thioredoxin reductase, a member of the thioredoxin-thioredoxin reductase antioxidant enzyme system. The selenocysteine component of these molecules is an integral part of their catalytic or active sites (Vendeland, S. C. (1995) Proc. Natl. Acad. Sci. 92:8749-8753; Zachara, B. A. (1992) J. Trace Elem. Electrolytes Health Dis. 6; 137-151; Mandel, S. J. et al. (1992) J. Clin. Endocrinol. Metab. 75:1133-1139; and Gladyshev, V. N. et al (1996) Proc. Natl. Acad. Sci. 93:6146-6151).
Levels of dietary selenium are found to regulate the enzymatic activities and the expression of the mRNAs of cellular GSHPx, phospholipid hydroperoxide glutathione peroxidase, and type I iodothyronine 5'-deiodinase. The recently isolated rat skeletal muscle selenoprotein W similarly responds to dietary selenium, although the levels of mRNA and protein do not respond in parallel. Selenoprotein W mRNA levels are relatively constant at 0.1 to 4.0 ppm selenium, but there is an 8-fold increase in the protein content which suggests additional regulation at the translational level. Like selenium-dependent GSHPx, rat skeletal muscle selenoprotein W can be isolated from muscle tissue by its binding to reduced glutathione which may imply a role in oxidation/reduction catalysis (Vendeland, S. C., supra).
Selenium-binding proteins have been implicated in cellular growth control and the protection from carcinogenesis and cancer. Antiproliferative effects of selenium have been demonstrated in brain tumor cell lines, and inhibition of DNA synthesis has been correlated with the level of selenium bound to proteins in cultured mouse mammary epithelial cells (Zhu, Z. et al. (1995) Biol. Trace Elem. Res. 49:1-7; Morrison, D. G. et al. (1988); Carcinogenesis 9:1801-1810).
The discovery of proteins related to rat skeletal muscle selenoprotein W, and the polynucleotides encoding them, satisfies a need in the art by providing new compositions useful in diagnosis and treatment of cancer and disorders of cell growth.